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Know Your Limits Application Report SNVA736–May 2016 Know Your Limits Frank De Stasi ABSTRACT This paper describes some of the more important features of current limit operation and how it effects the user application. The need for both overload and short circuit protection is highlighted along with some of the typical methods for providing this protection in DC/DC converters. Contents 1 Introduction ................................................................................................................... 2 2 Why Current Limit? .......................................................................................................... 2 2.1 Simple Current Limit................................................................................................ 2 2.2 Fold-Back Current Limit............................................................................................ 3 2.3 Bench-Top Active Loads........................................................................................... 5 2.4 Testing Current Limit Behavior.................................................................................... 6 3 Step-Down (Buck) DC/DC Converters .................................................................................... 7 3.1 Current Limit......................................................................................................... 7 3.2 Short Circuit Protection ............................................................................................ 9 3.3 Negative Output Voltage ......................................................................................... 10 3.4 Starting With Large Output Capacitors ......................................................................... 11 4 Step-Up (Boost) DC/DC Converters ..................................................................................... 12 5 Inverting Buck-Boost (Flyback) DC/DC Converters.................................................................... 13 6 Conclusion .................................................................................................................. 14 7 References .................................................................................................................. 15 List of Tables 1 Parameters for Maximum Current Limit Example ....................................................................... 8 2 Further Reading ............................................................................................................ 15 3 A Sample of DC/DC Converters.......................................................................................... 15 SNVA736–May 2016 Know Your Limits 1 Submit Documentation Feedback Copyright © 2016, Texas Instruments Incorporated Introduction www.ti.com 1 Introduction All voltage regulators require some form of current limitation to protect the converter components and/or the input supply. Although the basic idea of providing a current limit appears simple, when applied to DC/DC converters confusion can sometimes arise. This paper will help to explain what the different types of current limit strategies are and how they are used in the basic DC/DC converter topologies. 2 Why Current Limit? A voltage regulator is designed to regulate an output voltage against variations in input voltage, temperature, and load current. Since the regulator is composed of "real life" components, as is every source of input power, some limit on the amount of current that can be supplied by the regulator must be imposed. All voltage regulators specify the rated maximum load current in their data sheets; usually on the front page. As an example, the LM43603 can supply a maximum of 3A of load current, under most conditions. Once the load exceeds this value, the output voltage will drop. There are several reasons why every practical voltage regulator requires some kind of current limit. The most important is that circuit components and wiring all have an inherent maximum current capability. When this limit is exceeded the component will overheat and possibly be destroyed. For a DC/DC converter the inductor also imposes another limit on the current. Since these power inductors invariably use a metallic core, the maximum allowable current is limited by the saturation characteristics of the core material. If the core saturates, the inductance will drop drastically causing very high di/dt with probable damage to the regulator. 2.1 Simple Current Limit The graph in Figure 1 shows a typical current limit profile with three load characteristics superimposed. The solid black line represents the regulator output profile, while the dotted lines represent three different load resistance. The characteristic in this figure is somewhat ideal, especially in the current limiting region, but it serves to illustrate the basic concept of what is sometimes called a "brick-wall" type of current limit. A perfect constant voltage / constant current regulator would display horizontal and vertical lines in the two regions, respectively. In order to ensure that the regulator will provide its rated output current, the current limit must be set greater than this value by some margin. A 5V linear regulator, rated for 1A maximum load current and a current limit of 1.5A is shown in the figure. The intersection of the regulator characteristic and the load resistance shown by the dashed blue line (5Ω), represents the full load operating point; providing a regulated 5V at 1A to the load. The 2Ω load resistance represents an "overload". The regulator can only source 1.5A into the 2Ω load, giving an output voltage of only 3V rather than the specified 5V. The 0Ω load is termed a "short circuit"; here the output current is 1.5A while the output voltage is 0V. Note that for both an overload and a short circuit, the output current delivered to the load is 1.5A. The output current in this region is necessarily greater than the maximum load current rating of the device. For a linear regulator the output current in the limiting region will be equal to the specified current limit for the device; this is not true for a switching regulator. 2 Know Your Limits SNVA736–May 2016 Submit Documentation Feedback Copyright © 2016, Texas Instruments Incorporated www.ti.com Why Current Limit? Output Voltage 5V,1A 5 Constant 5V Voltage 3V,1.5A 2 2.5V Current Limiting 0V,1.5A 0 Load Current 0 1A 1.5A —Rated Load Current“ —Maximum Output Current“ or —Current Limit“ Figure 1. Brick-wall Current Limit Profile 2.2 Fold-Back Current Limit Although it may seem trivial, the distinction between an overload and a short circuit is very important for both linear and switching power converters. The characteristic shown in Figure 2 is much more commonly found in voltage regulators and is know as "current fold-back" limiting. The distinction between an overload and a short circuit is much clearer in this figure. For an overload of 2.7Ω, the output current is 1.1A at 3V. For a short circuit of 0Ω, the output current is 0.5A at 0V. Note that the short circuit current is much lower than the maximum output current. The most obvious reason to employ this type of profile is to limit the power dissipation in the regulator. Figure 3 illustrates this point with two different linear regulators. The device at the top of the figure uses the profile shown in Figure 1, while that at the bottom uses the profile shown in Figure 2. The difference in power dissipation between the two regulators is very apparent. Note that both regulators can supply a maximum of 1A to the load. This method is also used to protect the pass transistor in the regulator from moving outside of its safe-operating-area (SOA). Finally, using a fold-back type profile will limit the short circuit current to a small value while still allowing the regulator to provide its full rated output current. This can be very important if continuos short circuit faults need to be accommodated. Output Voltage 5V,1A 5 Constant Voltage 3V,1.1A 5V 2.7 3V Current Foldback 0V,0.5A 0 Load Current 0 0.5A 1A 1.5A —Short Circuit Current“ —Rated Load Current“ —Maximum Output Current“ or —Current Limit“ Figure 2. Fold-back Current Limit SNVA736–May 2016 Know Your Limits 3 Submit Documentation Feedback Copyright © 2016, Texas Instruments Incorporated Why Current Limit? www.ti.com PD = (12V ± 0V) × 1.5A = 18W Output Shorted VIN VOUT Brick Wall 1.5A 12V PD = (12V ± 0V) × 0.5A = 6W Output Shorted VIN VOUT Fold-back 0.5A 12V Figure 3. Short-circuited Regulator There are some pitfalls with fold-back type current limiting. If the short-circuit current is set too low, the regulator may not start up with certain loads. This is illustrated in Figure 4. Some active loads, such as a bench-top electronic load, will have a characteristic as shown by the dashed blue line in the figure. This characteristic starts at 0A at 0V and increases "resistively" up to the knee voltage. At that point the load current becomes constant. This is typical of many real loads, as well. In the top figure, the load profile is within the current limit profile of the regulator. In this case the regulator will start up correctly, since the current provided by the regulator is always greater than required by the load for all voltages. The bottom figure shows a case where the regulator can not start up the load. As the voltage ramps up from 0V, a point is reached where the load profile intersects with the current limit profile. At that point the regulator can not supply more current, at that voltage, and the regulator gets stuck at that intersection. In the
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